Nano-structures, especially nano-tubes and buckyballs, have been the subject of extensive research; they have remarkable tensile strength and exhibit varying electrical properties, such as superconducting, insulating, semi-conducting or conducting, depending on their helicity, and are thus utilizable as nanoscale wires and electrical components. The electrical conductivity is as high or higher than copper, thermal conductivity as high as diamond, and the tensile strength of these structures can be 100 times greater than steel, leading to structures that have uses in space, and that are believed to have applications as diverse as the formation of field-effect transistors and nano-motors. Indeed, there are those who believe nano-tubes and other nano-scale structures can be the solution to the hydrogen storage issues bedeviling the nascent hydrogen fuel cell industry, since hydrogen can be adsorbed on their surface.
When referring to nano-structures, what is meant is a structure which is, on average, no greater than about 1000 nanometers (nm), e.g., no greater than about one micron, in at least one dimension. Therefore, in the case of a nano-scale plate, the thickness (or through-plane dimension) of the plate should be no greater than about 1000 nm, while the plane of the plate can be more than one millimeter across; such a nano-plate would be said to have an aspect ratio (the ratio of the major, or in-plane, dimension to the minor, or through-plane, dimension) that is extremely high. In the case of a nano-tube, the average internal diameter of the tube should be no greater than about 1000 nm (thus, with a length of up to a millimeter (mm), the aspect ratio of nano-tubes is also extremely large); in the case of a buckyball, the diameter of the buckyball, such as the truncated icosahedron (the shape of a 60-carbon buckyball), should be no greater than about 1000 nm. A minor dimension of the nano-structure (for instance, the thickness of a nano-scale plate or the internal diameter of a nano-tube), should preferably be no greater than about 250 nm, most preferably no greater than about 20 nm.
Unfortunately, the production of commercial-scale quantities of nano-structures is expensive, laborious and time-consuming, to the extent that doing so is not considered feasible. Production processes currently employed include high pressure carbon monoxide conversion (HiPCO), pulsed-laser vaporization (PLV), chemical vapor deposition (CVD) and carbon arc synthesis (CA). None of these processes is considered adequate in the long term.
Natural graphite is formed of layered planes of hexagonal arrays or networks of carbon atoms, with extremely strong bonds within the layers, and relatively weak bonding between the layers. The carbon atoms in each layer plane (generally referred to as basal planes or graphene layers) are arranged hexagonally such that each carbon atom is covalently bonded to three other carbon atoms, leading to high intra-layer strength. However, the bonds between the layers are weak van der Waals forces (which are less than about 0.4% of the strength of the covalent bonds in the layer plane). Accordingly, because these inter-layer bonds are so weak as compared to the covalent intra-layer bonds, the spacing between layers of the graphite particles can be chemically or electrochemically treated so as to be opened up to provide a substantially expanded particle while maintaining the planar dimensions of the graphene layers.
It is this characteristic of natural graphite which is exploited in the production of sheets of compressed particles of exfoliated graphite (often referred to in the relevant industry as “flexible graphite”), which is used in the production of, inter alia, gasket materials, fuel cell components, electronic thermal management articles and devices, etc. As taught by Shane et al. in U.S. Pat. No. 3,404,061, natural graphite flakes can be intercalated by dispersing the flakes in a solution of a mixture of nitric and sulfuric acids. After intercalation, the flakes can be drained and washed, and are then exposed to temperatures, such as from about 700° C. to about 1000° C., with a high temperature of about 1200° C., which causes the flakes to expand in an accordion-like fashion in the direction perpendicular to the planes of the particle, by an amount that can be greater than 80 times, and as much as about 1000 times or greater, to form what are commonly called “worms.” These worms can then be formed in to sheets, even without the presence of binders, which can be formed, cut, molded and otherwise deformed.
Additional processes for the production of these sheets of compressed particles of exfoliated graphite are taught by, for instance, Mercuri et al. in U.S. Pat. No. 6,432,336, Kaschak et al. in International Publication No. WO 2004/108997, and Smalc et al. in U.S. Pat. No. 6,982,874. The unique directional properties of natural graphite (while graphite is commonly referred to as anisotropic, from a crystallographic standpoint, graphite should more properly be referred to as orthotropic or exhibiting transverse isotropy; in the plane of sheet, it is isotropic in two directions along the plane) provide sheets of compressed particles of exfoliated graphite having directional electrical and thermal characteristics, where conductivity is substantially higher along the plane of the sheet as opposed to through the sheet, is leveraged in the production of thermal management articles and fuel cell components.
The intercalation process described above functions to insert a volatile species between the layer planes of the graphite flake which, when exposed to high temperatures, rapidly volatilizes, causes separation of the layers and, consequently, exfoliation. Typical intercalation of graphite for the production of sheets of compressed particles of exfoliated graphite is Stage VII or greater Stage value. The Stage Index is a measure of the average number of graphene layers between each “gallery” (the space between graphene layers in which the chemical intercalant is inserted), rounded to the nearest whole number. Therefore, in Stage VII intercalation, there are, on average, less than 7.5 graphene layers between each gallery. In Stage VIII intercalation, there are, on average, at least 7.5 graphene layers between each gallery.
The Stage Index of an intercalated graphite flake can be determined empirically by x-ray diffraction to measure the “c” lattice spacing (the spacing between any three graphene layers), where a spacing of 6.708 indicates (Å) represents a non-intercalated graphite flake and over 8 Å indicates an intercalated flake with Stage I intercalation (on average, only one graphene layer separating each gallery, or as complete intercalation as possible).
Processes for preparing lower intercalation Stages (more specifically, Stage III and lower) are known. For instance, Kaschak et al. (International Publication No. WO 2004/108997) described a process for preparing Stage V (i.e., intercalation between, on average, every fifth graphene layer) or lower intercalation using supercritical fluids. Other systems for preparing intercalated graphite flakes having Stage III or higher degree intercalation (that is, intercalation to Stage I, II or III) using methanol, phosphoric acid, sulfuric acid, or simply water, combined with nitric acid in various combinations, are known, for both “normal” or “spontaneous” intercalation and electrochemical intercalation.
For instance, an admixture of up to 15% water in nitric acid can provide Stage III or II spontaneous intercalation and Stage I electrochemical intercalation; for methanol and phosphoric acid, an admixture of up to 25% in nitric acid can provide Stage II spontaneous intercalation and Stage I electrochemical intercalation. The chemical or electrochemical potential of the intercalant critically effects the thermodynamics of the process, where higher potential leads to a lower stage number (i.e., a greater degree of intercalation), while kinetic effects such as time and temperature combine to define processes which can be of commercial importance.
What is desired, therefore, is a process for preparing nano-structures in a cost-effective and commercially feasible manner. The desired process will enable the production of nano-structures, whether nano-tubes, buckyballs or nano-plates, in quantities sufficient for industry-scale uses without the requirement of exotic equipment, unusual raw materials or extreme process parameters.